Information recording discs to perform storing and reading information by converging rays of light such as laser beam and the like on recording layers of the discs are known, said information recording discs being prepared by laminating at least two plastic disc substrates, each having a recording layer on one surface thereof, directly or via spacers with adhesives so that the recording layers face to each other. A magnet clamping system is one of clamping systems for clamping such information recording discs of the type as referred to above.
In the accompanying drawings, FIG. 4 is a sectional view of an information recording disc, showing the state wherein the disc is going to be clamped by a conventional magnet clamping system, for example, such as disclosed in Japanese Patent L-O-P Publn. No. 167663/1987. In FIG. 4, 1 is an information recording disc, which is so assembled that two transparent plastic disc substrates 2 are laminated together via an inner peripheral spacer 4 and an outer peripheral spacer (not shown) so that recording layers 3 face to each other, hubs 6 are fitted individually from above and below to a center hole 5 of the two disc substrates 2 and bonded with adhesive layers 7 to the top and bottom surfaces of the laminated disc substrate. The information recording disc 1 thus assembled is contained in a cartridge 8 and fitted onto a turn table 9 to effect storing and reading of information. In the turn table 9, it is so designed that a concavity 10 equipped with a magnet 11 and a yoke 12 is provided so as to attract and set the hubs 6 on the turn table 9 and said hubs 6 are positioned by inserting a spindle 13 into center holes of the hubs 6, thereby setting and holding the information recording disc 1 on the turn table 9.
The information recording disc 1 set on the turn table 9 in the manner now described is rotated, and information is stored in or read from the recording layers 3 by irradiation from the bottom with a laser beam or the like through the laminated disc substrate 2.
The information recording disc 1 comprises two disc substrates 2 laminated together, each having a recording layer, and is assembled and used in the manner as illustrated above. When an amount of information to be recorded is small, however, it is taken into consideration that the information is stored or read by means of an information recording disc comprising one disc substrate having a recording layer, said information recording disc being used for storing or reading information likewise after containing it in the cartridge 8.
FIG. 5 is a sectional view of a conventional single substrate type information recording disc which is one of the two parts obtained by simply dividing the information recording disc of FIG. 4 into two halves. That is, this information recording disc 1 is assembled by inserting an insertion portion 6a of a hub 6 having a flange portion 6b into a center hole of a disc substrate 2 having a recording layer 3 from the side opposite to the recording layer 3, and bonding the disc substrate 2 with an adhesive layer 7 to the flange portion 6b. In the same manner as in the case of the information recording disc of FIG. 4, the information recording disc 1 of FIG. 5 is used likewise for storing or reading information after inserting the hub 6 into a concavity 10 equipped with a magnet 11 and a yoke 12 of a turn table 9 to attract the hub 6 thereto, and inserting a spindle 13 into a center hole 14 of the hub 6 to position and set the information recording disc on the turn table 9.
In the conventional single substrate type information recording disc as mentioned above, however, there were such problems that because the flange portion 6b of the hub 6 is positioned at the side of the disc substrate 2 opposite to the recording layer 3, in a case wherein a clamping area 15 on the concavity 10 of the turn table 9 is small in diameter, the diameter of the flange portion 6b becomes small and an adhesive surface thereof also becomes small, and in a case wherein the diameter of the clamping area 15 is the same as or smaller than that of the center hole 5, the flange portion 6b disappears, so adhesion of the disc substrate 2 to the hub 6 is attained only at the edge face of the insertion portion 6a, thus in either case the adhesion strength between the disc substrate 2 and the hub 6 decreases. Further, there was a similar problem when the clamping area formed on the concavity 10 of the turn table 9 is shallow, and when no concavity 10 is present at all in the clamping area 15, that is, when the back surface of the hub 6 and that of the disc substrate 2 are on the same level, there was also such problem that the disc substrate 2 is bonded to the hub 6 only at the edge face thereof.